Poly(ethylene terephthalate)-based resins, which are commonly referred to in the industry simply as “PET” even though they may and often do contain minor amounts of additional components, have widely been used to make containers for carbonated soft drinks and water due to being light weight and having an excellent combination of mechanical and gas barrier properties. In these traditional applications, the contents of the container are at ambient temperature or cold when introduced into the container (“cold-fill process”). Recently, PET containers have been used beyond such applications and have been used in applications for beverages such as juices, flavored sports drink, and teas, in which the contents of the container are at an elevated temperature when introduced into the container (“hot-fill process”).
The hot-fill process subjects the containers to a high temperature treatment when the beverage is placed in the containers. This high temperature treatment causes unacceptable shrinkage or deformation of PET containers that are produced under conventional preform injection molding and container stretch blow molding processes used to make cold-filled containers. For PET containers to be used in hot-fill processes, several solutions have been developed to eliminate shrinkage and deformation. Such solutions include converting preforms into containers using a heat-set stretch blow molding (SBM) process, designing bottles with special vacuum panels, using special grades of PET resins or combinations of these. Preforms are test tube shaped articles prepared by injection molding of the PET using technology well know in the art.
In the heat-set SBM process, preforms are heated to a temperature of about 90° C. to about 140° C., which is above the glass transition temperature of the polymer, and then placed into molds heated at temperatures of about 60° C. to about 200° C. Pressurized gas is injected or blown into the heated preforms expanding and stretching the preform onto the mold surface (“stretch blowing step”). Differences between the heat-set and the non-heat-set SBM processes are that, in heat-set processes, heated molds are used rather than ambient or cold molds of about 10° C., the preform is heated a sufficient time to allow for the preform to reach a substantially uniform temperature prior to stretching, and the stretch blowing step is slowed relative to typical speed used to manufacture non-heat-set SBM containers. The speed is slowed to allow for a long contact time between the blow mold and the forming container. The heat-set SBM process uses more energy and requires more time than a non-heat-set SBM process, thus increasing the manufacturing costs of heat-set containers. Containers made by conventional heat-set SBM processes can be hot-filled to a temperature of about 85° C. without severe shrinkage.
The hot-fill temperature requirements, however, are increasing and, in some instances, to beyond the normal PET glass transition temperature. Since the hot-fill temperature is related to both crystallinity of the container sidewall and the glass transition temperature of the polyester, several methods have been used to achieve even higher hot-fill temperatures. One method is the use of special heat-set SBM processes. For example, a double-blow heat-set SBM process enables high crystallinity of more than thirty five percent based on density measurement to develop in the sidewall allowing for hot-filling to above 90° C. However, in this process the manufacturing speed is dramatically slower because of the double stretch blowing step and results in an increased cost to produce the higher temperature hot-fill containers.
Another method uses specially designed resins that have property or co-monomer modification. These special PET resins have higher glass transition temperatures, can achieve higher crystallinity during the heat-set SBM process or a combination of both. In one example, the molecular weight of the PET resin is increased to reduce both preform gravitational deformation and the natural stretch ratio of the PET resin. However, increasing molecular weight increases the manufacturing cost of the PET resin and often increases the preform injection molding cycle time due to increased injection temperature of the more viscous materials.
Several co-monomers have been used to modify PET resins to obtain higher glass transition temperatures, including diacids such as naphthalene dicarboxylic acid (NDA), diols such as 1,4-cyclohexanedimethanol (CHDM) or a combination of both. The total modification for NDA is typically above 5 mole percent to get the desired results. This high level of modification, however, changes the stretching behavior and the crystallization behavior of the polyester such that very thick side-walled preforms have to be designed and/or the process has to be slowed down to achieve the high degree of crystallinity needed. In another specially designed resin, the PET resin has reduced co-monomer content such that the polymer is essentially a homo-polymer except for the presence of naturally occurring diethylene glycol at about 2.8 mole percent. Although the crystallization rate is dramatically improved and high crystallinity can be achieved, the crystallization is too fast such that the preforms tend to be hazy and thus the containers are hazy. The containers also do not achieve the desired optimal material distribution due to the difficulty in blow molding the crystallized preforms resulting in undesirable containers. In still another specially designed resin, only the diol component of the PET resin is modified with 1-4 mole percent CHDM and 1-4 mole percent diethylene glycol (DEG) and the PET resin contains reheat additives such as carbon black, iron oxide, antimony metals, and the like. Reheat refers to heating the preform prior to the stretch blowing step. The term “reheat” is used in the industry because at this stage the polymer has previously been heated during formation of the preform and is now undergoing reheat to form the container.
To achieve the mechanical properties needed to survive the high temperature and subsequent vacuum conditions of the hot-filling process, PET containers are designed to have very thick side walls. Such thick-walled bottles are blown from thick-walled preforms. Since the injection molding cooling time is proportional to the square of the preform sidewall thickness, heat-set containers tend to have much higher cycle time, i.e. much lower productivity, than non-heat-set containers during injection molding. Time to reheat the thick-walled preform in the blow molding process is also increased. Further, the thick-walled bottles also mean more material has to be used to produce the bottles. This can cause environmental awareness in source reduction of both material and energy usage.
Thus, there is a need in the art for PET resins that can be used to make hot-fill containers capable of being filled at 85° C. or higher filling temperatures, that have modifications low enough that the stretch ratio and the crystallization rate of the PET are not increased and the corresponding preforms are clear, that can be utilized in conventional high speed heat-set SBM processes, and that can be used to produce hot-fill containers with reduced weight resulting in reduced energy usage via reduced cycle time. Accordingly, it is to the provision of such that the present invention is directed.